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Carbonation of the Oman Ophiolite during subduction and emplacement

University of British Columbia

Fully carbonated peridotite, known as listvenite, is well exposed along the basal thrust of the Oman ophiolite. Outcrops of listvenite are a fossilized record of fluid-rock interactions that resulted in the storage of billions of tonnes of CO₂, providing researchers insight into an important global carbon sink and a potential natural analogue to in situ carbon sequestration. The goal of this thesis is to investigate the formation of listvenite using samples from the Oman Drilling Project to 1) characterize the petrology and geochemistry of listvenite, 2) determine the mineral reaction sequence and the physical and chemical conditions required and 3) constrain the potential sources of CO₂ bearing fluids. Three carbonated lithologies were observed: 1) serpentine with variable amounts magnesite and dolomite veining (ophicarbonate); 2) serpentine with abundant vein and matrix hosted magnesite, dolomite, talc and quartz (talc-ophicarbonate); and 3) intergrown magnesite, dolomite and quartz (listvenite). With the exception of H₂O, CO₂ and variable amounts of Ca, the major element geochemical composition of all three lithologies approximates unaltered peridotite. Fluid mobile trace elements such as Cs, Rb and Sr are enriched in listvenite to over 100-times that of unaltered peridotite and correlate with increasing δ¹³C values. δ¹³C values increase with carbonation from ~-7 δ¹³C in ophicarbonate samples through to ~+2 δ¹³C in listvenite, representing the progressive dilution of mantle-derived carbon with marine-derived carbon. Detailed petrography and thermodynamic phase equilibria modelling demonstrate that listvenite formed via a series of mineral reactions resulting in the breakdown of serpentine and the formation of magnesite, dolomite, talc and quartz. Crosscutting relationships between carbonate minerals and deformation structures, in combination with oxygen isotopic exchange thermometry, reveal that carbonation reactions were driven by at least two pulses of CO₂-bearing fluids that originated from, or interacted with, the metamorphic rocks below. The first pulse is associated with deformed matrix- and vein-hosted magnesite carbonation at 150-250 ℃, which potentially occurred pre-syn obduction at shallow depths within a subduction zone. This was followed by a later pulse of calcium-rich fluids associated with late undeformed dolomite carbonation at 50-100 ℃, which potentially occurred syn-post obduction during thrusting over carbonate shelf sediments.

Text

2021-01-27

Attribution-ShareAlike 4.0 International

http://hdl.handle.net/2429/77182

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-sa/4.0/